In PODC 2003, Park, Chong, Siegel and Ray [22] proposed an optimistic protocol for fair exchange, based on RSA signatures. We show that their protocol is totally breakable already in the registration phase: the honest-but-curious arbitrator can easily determine the signer's secret key.On a positive note, the authors of [22] informally introduced a connection between fair exchange and "sequential two-party multisignature schemes" (which we call two-signatures), but used an insecure two-signature scheme in their actual construction. Nonetheless, we show that this connection can be properly formalized to imply provably secure fair exchange protocols. By utilizing the state-of-the-art non-interactive two-signature of Boldyreva [6], we obtain an efficient and provably secure (in the random oracle model) fair exchange protocol, which is based on GDH signatures [9].Of independent interest, we introduce a unified model for non-interactive fair exchange protocols, which results in a new primitive we call verifiably committed signatures. Verifiably committed signatures generalize (non-interactive) verifiably encrypted signatures [8] and two-signatures, both of which are sufficient for fair exchange.
In PODC 2003, Park, Chong, Siegel and Ray [22] proposed an optimistic protocol for fair exchange, based on RSA signatures. We show that their protocol is totally breakable already in the registration phase: the honest-but-curious arbitrator can easily determine the signer's secret key.On a positive note, the authors of [22] informally introduced a connection between fair exchange and "sequential two-party multisignature schemes" (which we call two-signatures), but used an insecure two-signature scheme in their actual construction. Nonetheless, we show that this connection can be properly formalized to imply provably secure fair exchange protocols. By utilizing the state-of-the-art non-interactive two-signature of Boldyreva [6], we obtain an efficient and provably secure (in the random oracle model) fair exchange protocol, which is based on GDH signatures [9].Of independent interest, we introduce a unified model for non-interactive fair exchange protocols, which results in a new primitive we call verifiably committed signatures. Verifiably committed signatures generalize (non-interactive) verifiably encrypted signatures [8] and two-signatures, both of which are sufficient for fair exchange.
In PODC 2003, Park, Chong, Siegel and Ray [22] proposed an optimistic protocol for fair exchange, based on RSA signatures. We show that their protocol is totally breakable already in the registration phase: the honest-but-curious arbitrator can easily determine the signer's secret key.On a positive note, the authors of [22] informally introduced a connection between fair exchange and "sequential two-party multisignature schemes" (which we call two-signatures), but used an insecure two-signature scheme in their actual construction. Nonetheless, we show that this connection can be properly formalized to imply provably secure fair exchange protocols. By utilizing the state-of-the-art non-interactive two-signature of Boldyreva [6], we obtain an efficient and provably secure (in the random oracle model) fair exchange protocol, which is based on GDH signatures [9].Of independent interest, we introduce a unified model for non-interactive fair exchange protocols, which results in a new primitive we call verifiably committed signatures. Verifiably committed signatures generalize (non-interactive) verifiably encrypted signatures [8] and two-signatures, both of which are sufficient for fair exchange.
In PODC 2003, Park et al. [32] first introduce a connection between fair exchange and sequential two-party multi-signature scheme and provide a novel method of constructing fair exchange protocol by distributing the computation of RSA signature. This approach avoids the design of verifiable encryption scheme at the expense of having cosigner store a piece of prime signer's secret key. Dodis and Reyzin [20] showed that the protocol in [32] is totally breakable in the registration phase, then presented a remedy scheme which is provably secure in the random oracle model, by utilizing Boldyreva non-interactive two-party multi-signature scheme [8]. Security in the random oracle model does not imply security in the real world. In this paper, we provide the first two e#cient committed signatures which are provably secure in the standard complexity model from strong RSA assumption. Then we construct e#cient optimistic fair exchange protocols from those new primitives.
Fair exchange protocols aim to allow two parties to exchange digital items in a fair manner. Optimistic fair exchange (OFE) is a kind of protocols that solves the fair exchange problem with the help of a trusted third party (TTP), usually referred to as an `arbitrator'. The participation of the arbitrator is only required when there is a dispute between the exchanging parties. In the literature, the highest level of security of optimistic fair exchange is the multi-user security in the chosen-key model, proposed by Huang, Yang, Wong and Susilo in CT-RSA 2008. They showed that an efficient optimistic fair exchange scheme, secure in this sense, can be constructed generically from a conventional signature and a ring signature. In particular, the underlying ring signature is required to be unforgeable under an adaptive attack, against a static adversary in the 2-user setting.\nConcurrent signatures, introduced by Chen, Kudla and Paterson in Eurocrypt 2004, allow two parties to produce two ambiguous signatures until an extra piece of information, the keystone, is released by one of the parties. Upon the release of the keystone, the ambiguity will be revoked and the signatures bind to their true signers concurrently. Concurrent signatures, however, are known to fall just short of fully solving the long standing fair exchange of signatures problem. The price for not requiring any TTP is that the party who holds the keystone always has an advantage over the other party in controlling when and whether the protocol completes or not.\nIn this thesis, the fair exchange of digital signatures problem is studied. In the first part of this thesis, the OFE security is strengthened and a more practical model which is called enhanced chosen-key model is proposed. Unlike the existing multiuser setting and chosen-key model, an adversary in the enhanced chosen-key model is further provided with the signing oracle that outputs full signatures generated by the signer, and may even be allowed to have access to the arbitrator's secret key. The necessity for the new model is demonstrated. It is shown that two existing methodologies for constructing optimistic fair exchange schemes remain valid in the enhanced chosen-key model.\nNext, the efficiency of OFE constructions is improved. A model for 2-user ring signatures called `unforgeability against restricted adaptive attacks' is proposed and it is shown that the new ring model is strictly weaker than the model of unforgeability under an adaptive attack, against a static adversary in the 2-user setting. Based on the finding that 2-user ring signatures secure in this weaker model will suffice to guarantee the security of the resulting OFE schemes in the enhanced chosenkey model, the most efficient OFE scheme whose security does not rely on random oracles is proposed.\nNew situations in OFE are also investigated, and the applications of OFE are extended. The scenarios where the digital items to be exchanged are threshold signatures are considered, and the notion of threshold-oriented OFE (TOFE) together with a security model and an efficient concrete scheme is proposed. It is identified that in some scenarios, it is required that prior to the completion of the protocol, no information about the actual exchange should be revealed to an outsider. To achieve this purpose, the notion of perfect ambiguous optimistic fair exchange (PAOFE) is introduced, the security model for PAOFE is proposed and a generic construction based on existing primitives is offered. OFE in the attribute-based setting is considered and the notion of attribute-based optimistic fair exchange (ABOFE) is proposed. The security model and a construction for ABOFE are presented as well.\nFinally, concurrent signatures is revisited. It is showed that theoretically the party who controls the keystone in a concurrent signature scheme has more advantages than previously thought. In particular, the notion is examined and the advantages are classified into three levels.It is demonstrated that concurrent signatures may be abused in practical applications if one fails to take these advantages into account.
The problem of fair exchange is a fundamental problem in secure electronic transactions and digital rights management. Park et al. (2003) presented an optimistic fair-exchange protocol based on RSA. Dodis and Reyain (2003) analyzed the vulnerability of Park's scheme and presented an optimistic fair-exchange protocol based on GDH. This paper point out that Dodis and Reyain's scheme is also insecure and inefficient. This work presents a multisignature scheme based on DSA, and describes a novel method of constructing very efficient fair-exchange protocols based on improved DSA signatures. The protocols in this paper are more secure and efficient.
Consider a scenario in which one party A consisting of a number of manufactories would like to sign a contract with another party B who is a selling company so that they can unify prices to avoid over competition. This may be done via Internet to reduce travelling cost. To cater for such applications, we propose the concept of optimistic fair exchange of distributed signatures (OFEDS) which allows an authorized set of signers to fairly exchange signatures with a verifier, and a trusted arbitrator will intervene in the protocol only when there are disputes between two parties. We consider the security requirements for OFEDS, in which besides the standard security requirements for existing optimistic fair exchange protocols, robustness is introduced to ensure that OFEDS can be successfully performed even when there exist some dishonest signers. We instantiate a non-interactive construction of OFEDS based on the Computational Diffie-Hellman (CDH) assumption in the random oracle model. Our work shows that for any general monotone access structure, there exists a CDH-based OFEDS protocol if all the involved signers are honest.
A fair exchange scheme is a protocol by which two parties Alice and Bob exchange items or services without allowing either party to gain advantages by quitting prematurely or otherwise misbehaving. To this end, modern cryptographic solutions use a semi-trusted arbitrator who involves only in cases where one party attempts to cheat or simply crashes. We call such a fair exchange scheme optimistic. When no registration is required between the signer and the arbitrator, we say that the fair exchange scheme is setup free. To date, the setup-free optimist fair exchange scheme under the standard RSA assumption was only possible from the generic construction of [12], which uses ring signatures. In this paper, we introduce a new setup-free optimistic fair exchange scheme under the standard RSA assumption. Our scheme uses the GQ identity-based signature and is more efficient than [12]. The construction can also be generalized by using various identity-based signature schemes. Our main technique is to allow each user to choose his (or her) own “random” public key in the identitybased signature scheme.